The war was fought over slavery, the Confederacy had seceded to protect it, and on this campaign, more than almost anywhere else, the abstraction turned into something you could watch happen on the ground. As the Army of the Tennessee pushed through the Mississippi Valley, slavery dissolved in front of it. The institution the South went to war to preserve came apart, plantation by plantation, in the army’s path.

It did not come apart because Grant freed people, exactly. It came apart because enslaved people freed themselves. Historians of the valley stress the point: the enslaved men and women here, as the Mississippi Encyclopedia puts it, "more often already had taken steps to secure their own emancipation," young men slipping away in small groups toward Union lines, families walking off the plantations toward the soldiers, rather than waiting to be liberated. Thousands self-emancipated as the army advanced, until the surrender itself ended slavery in the city: "July 4, 1863: Slavery Ends in Vicksburg" is the actual title the National Park Service gives the day. The campaign that split the Confederacy also split open slavery along the river, and the people doing the splitting were, in large part, the enslaved themselves.

The army had to figure out what to do with the tide of people coming to it, and the answer was both a lifeline and a hard place. Grant established refugee camps, "contraband" camps in the awkward legal language of the time, the word the Union used for enslaved people who reached its lines, across the Mississippi Valley, sheltering thousands of freedpeople. Conditions in them were often brutal: families who had walked to freedom found themselves crowded into camps where measles and smallpox tore through the tents, where Union soldiers’ contempt and outright violence followed them, and where many were put back to labor, for wages now, but on the same leased plantations, under overseers, picking the same cotton. Freedom, on the ground, often looked like survival under guard. But the camps were also where free Black life along the river began to organize itself, and they were a recruiting ground. A senior Union officer, Adjutant General Lorenzo Thomas, came down the valley specifically to enlist freedmen into the army; the men who joined from the Mississippi Valley numbered in the tens of thousands, enough to raise dozens of new Black regiments, on the order of seventy of them by the end of the war. The men the South had owned were now being handed rifles to fight the South.

Whether those men would fight was a question a lot of people, North and South, claimed to know the answer to. On June 7, 1863, while the siege ground on, a Confederate force struck a Union supply depot on the Louisiana bank at Milliken’s Bend, about fifteen miles above Vicksburg. The depot was held largely by the African Brigade, four regiments of newly recruited, formerly enslaved men, days into uniform, barely trained and poorly armed, stiffened by the white 23rd Iowa. At dawn a Texas brigade of roughly 1,500 under Brigadier General Henry McCulloch (South) came at them. What followed was some of the most savage hand-to-hand fighting of the entire war: bayonets and clubbed muskets across the levee and the cotton-bale barricades, men killing each other at arm’s length. Union gunboats on the river, the Choctaw and the Lexington, finally broke the attack with their heavy guns.

Union casualties at Milliken’s Bend ran to somewhere between roughly 490 and 650 (the figure is genuinely disputed across sources, with the Park Service citing around 652 and others closer to 492) against perhaps 185 Confederate. The 9th Louisiana, the formerly enslaved regiment at the center of it, lost something on the order of 68 percent of its strength, the greatest proportional loss of any Black regiment in the war; its dead were the most any single Union regiment lost on any one day of the whole Vicksburg campaign. They had stood, and fought hand-to-hand, and held until the gunboats came. Milliken’s Bend, together with the fights at Port Hudson and Fort Wagner that summer, shattered the claim that Black men would not or could not fight. The Secretary of War, Edwin Stanton, said the formerly enslaved soldier had "proved his manhood" in these battles.

And it was Lincoln who tied it all together, the river and the men who freed themselves to open it. In a public letter dated August 26, 1863, to James C. Conkling, written just after the victory, Lincoln bound the unblocked Mississippi directly to the Black soldiers who had helped win it. On the river, he wrote that the Father of Waters again went unvexed to the sea. On the soldiers, in the same letter, he was just as plain. They had to be fighting for freedom, he reasoned, because that was the only motive strong enough:

"If they stake their lives for us, they must be prompted by the strongest motive, even the promise of freedom. And the promise being made, must be kept."

Lincoln, letter to James C. Conkling, August 26, 1863

And he looked ahead to how they would be remembered: "there will be some black men who can remember that, with silent tongue, and clenched teeth, and steady eye, and well-poised bayonet, they have helped mankind on to this great consummation." The man in the White House had drawn the line himself, from the opened river straight back to the cause: slavery, and the people ending it.